1. Field of the Invention
The present invention relates to methods for forming phosphor surfaces of image display devices, such as cathode ray tubes (CRT), fluorescent display tubes (VFD), and field emission displays (FED), using luminescence of a phosphor generated by electron-beam emission, and more particularly, relates to a phosphor screen substrate, which has a phosphor layer and a metal film provided thereon, of an image display device and a manufacturing method of the phosphor screen substrate described above.
2. Description of the Related Art
Image display devices using luminescence by electron-beam emission have provided self-luminous bright display devices having superior color reproducibility, and cathode ray tubes (hereinafter referred to as “CRT”) have been used practically for these long years. In addition, concomitant with recent diversified information and higher density thereof, further improvements in performance and image quality and increase in screen size have been increasingly required for image display devices. Furthermore, concomitant with the recent aggressive trend toward energy saving and space saving, among various image display devices, a field emission display (hereinafter referred to as “FED”), which is a planar image display device, has particularly drawn attention.
In addition, in CRTs and high-voltage FED at an accelerating voltage of 5 kV or more, in order to effectively remove charges accumulated on a phosphor surface and to effectively reflect phosphor luminescence to a display screen, a metal film is generally formed on a phosphor layer by deposition. In addition, as a metal for forming the metal film, aluminum (Al) has been generally used since electrons are allowed to easily flow thereinto.
It is necessary for the metal film to have no irregularities thereon and to be uniform over the entire screen. The reason for this is that when an image is displayed on the screen, it is important that a display screen having superior white uniformity (hereinafter referred to as “Wu”) be formed. Secondly, in order to effectively use luminescence, the metal film preferably has the structure in which the luminescence is effectively reflected to the front side.
In the FED which is a planar image display device, when electrons at a high current density irradiate a phosphor, by this irradiation mentioned above, highly reactive gases are generated. Accordingly, the metal film is expected to inhibit the diffusion of these reactive gases into a vacuum container for protecting various constituent elements such as electron sources and partitions from being contaminated, and from this point of view, thirdly, it has been important that the metal film have a small number of pinholes therein.
In the FED, since a rear substrate provided with electron sources arranged in a matrix and wires for driving the electron sources and a front substrate provided with a phosphor layer thereon are disposed with a very small space, approximately 2 to 8 mm, provided therebetween, and a high voltage of approximately 2 to 18 kV is applied to this space, suppression of discharge generated between the substrates has been an important technical subject. From this point of view, fourthly, it has been important for the metal film formed on the phosphor surface to have a high withstand voltage structure in which discharges generated between the substrates can be suppressed and in which damages done to the substrates by discharges can be reduced as small as possible.
Although the mechanism of this discharge generation has not been understood well, as factors for causing discharges, which are estimated from an empirical point of view, for example, there may be mentioned projections on the substrate, dust approximately 5 μm in diameter, fine particles, scratches or cracks in the surface of a metal film formed by deposition (hereinafter referred to as “metal deposition surface”), and hangnails formed thereby. When discharge is once generated, wrinkles, sags, or liftings formed on the metal deposition surface are selectively damaged. Hence, as a phosphor surface having superior withstand voltage properties, dust and fine particles must not be present thereon, and in addition, scratches, hangnails, cracks, sags, and liftings must not be present on the metal deposition surface.
As a method for forming this metal film, a method comprising the steps of first forming a resin-made intermediate layer (hereinafter referred to as “resin interlayer”) on a phosphor surface so that the irregularities thereof is planarized thereby, then depositing a metal, and finally removing the resin interlayer by pyrolysis is generally used. For forming the resin interlayer, as a first method, for example, a method disclosed in Japanese Patent Laid-Open No. 7-13029 1 may be mentioned in which a film of a solvent-based lacquer is formed by spin coating. In particular, the method described above comprises the steps of coating a phosphor surface with an aqueous solution containing, for example, colloidal silica and a surfactant so that the irregularities on the phosphor surface are put in a sufficiently wet state; dissolving a resin, such as polymethacrylate, having superior pyrolyzable properties in a nonpolar solvent such as toluene or xylene together with a plasticizer; spraying the resin solution thud prepared onto the phosphor surface planarized in the wet state mentioned above so that oil in water (o/w) type droplets are placed on the phosphor layer; spreading the droplets by spin coating; and removing water and solvent components by drying.
As a second method, for example, there has been a method as disclosed in U.S. Pat. No. 3,582,390, which comprises the steps of applying an aqueous solution containing colloidal silica, a surfactant, and the like on a phosphor surface so as to put it in a sufficiently wet state, as is the method described above; directly coating the phosphor surface with an aqueous emulsion containing a resin, such as an acrylate copolymer, which has superior pyrolyzable properties; forming a thin film of the aqueous emulsion by spin coating; and removing a water component by drying so as to form a resin interlayer.
In both methods described above, since spin coating is used, when the spin rotation speed is increased while the phosphor surface is in a wet state prior to the formation of the resin interlayer, an infiltrating resin interlayer, that is, a resin interlayer which infiltrates between phosphor particles and is closely brought into contact therewith, can be formed, and hence a metal deposition surface having a high withstand voltage can be formed without any lifting, sags, and the like. However, according to experiments carried by the inventors of the present invention, when the spin rotation speed is merely increased, the degree of infiltration of the resin interlayer varies within an effective area and varies particularly between the central portion and the peripheral portion thereof, and as a result, a uniform phosphor surface having superior white uniformity is difficult to obtain. In addition, the phenomenon described above becomes more observable as the screen size is increased.
In recent years, the two methods described above have been primarily used; however, in addition to those described above, as a third method which can be applied particularly to a planar image display device, for example, there may be mentioned a method disclosed in Japanese Patent Laid-Open No. 2000-243270. The method mentioned above comprises the steps of forming a printing paste having appropriate rheological properties and containing a resin which is to be formed into a resin interlayer; and forming the resin interlayer by directly coating a phosphor screen substrate with this paste using a coating technique such as a screen printing or a doctor blade method. However, according to this method, the phosphor surface cannot be placed beforehand in a wet state for planarization, and hence drying of the paste after coating must be performed as quick as possible. Otherwise, the resin to be formed into the resin interlayer totally infiltrates between the particles of the phosphor, and a result, the resin cannot function as a resin interlayer since a meal film formed thereon may not have a continuous surface in some cases. Hence, although the method described above is used, it has been still difficult to form a resin interlayer having an appropriate degree of infiltration.
In each of the first to the third methods described above, after the resin interlayer is formed, Al is formed on the surface thereof by deposition; however, at the stage of forming the resin interlayer, methods for reducing the generation of discharge and for suppressing damages done onto the phosphor surface during discharge have not been disclosed at all. Accordingly, sags and liftings are likely to be formed on the metal deposition surface to be formed on the resin interlayer, and hence destruction of the metal deposition surface disadvantageously tends to occur during discharge.
Furthermore, as a fourth method, methods have been disclosed in Japanese Patent Laid-Open No. 2000-243271. In the publication described above, for example, there have been mentioned a method comprising the steps of depositing aluminum (Al) on a resin film having superior pyrolyzable properties, and then joining the resin film provided with Al to a phosphor surface by fusion or compression bonding; and a method comprising the steps of depositing a metal on a release film, applying a resin which is to be formed into a resin interlayer on the release film mentioned above by a printing technique or the like, then joining this composite film thus formed to a phosphor surface by fusion bonding, and removing the release film. However, in the methods described above, since a film provided with a metal such as Al deposited thereon is directly bonded to the phosphor screen substrate by thermal fusion, scratches or cracks are likely to be mechanically formed on the metal deposition surface, and as a result, problems may arise in that, for example, wrinkles are easily formed when the film is handled. Furthermore, when contraction of the film in fusion bonding, mechanical impacts generated during compression bonding, and the like are not appropriately taken into consideration, sags and liflings are likely to be formed on the metal deposition surface. As a result, problems may be encountered in that discharge occurs frequently at a low voltage, and the metal deposition film is seriously damaged during the discharge. In addition, according to the methods described above, since Al is deposited beforehand on the resin interlayer, it is more difficult to suppress the generation of discharge and damage done to the phosphor surface during discharge at the stage at which the resin interlayer is formed.